Name: Bitrus Atang

Reg No: AKP/ENG/EET/HND 2008/019

Course: Digital Communication III

Assignment:

Ques: Explain in details the principle of operation and application of:

i. Microwave System
ii. Radar System
iii. Satellite Communication

Instructions: Not less than Five pages.

Solution

Definition of Microwaves

Microwave frequencies range from 300 MHz to 30 GHz, corresponding to

wavelengths of 1 meter to 1 cm.  These frequencies are useful for terrestrial
and satellite communication systems, both fixed and mobile.  In the case of
point-to-point radio links, antennas are placed on a tower or other tall
structure at sufficient height to provide a direct, unobstructed line-of-sight
(LOS) path between the transmitter and receiver sites. In the case of mobile
radio systems, a single tower provides point-to-multipoint coverage, which
may include both LOS and non-LOS paths.  LOS microwave is used for both
short- and long-haul telecommunications to complement wired media such
as optical transmission systems.  Applications include local loop, cellular
back haul, remote and rugged areas, utility companies, and private
carriers.   Early applications of LOS microwave were based on analog
modulation techniques, but today’s microwave systems used digital
modulation for increased capacity and performance.
 

Principles and Operation

Microwave Link Structure.  The basic components required for operating a

radio link are the transmitter, towers, antennas, and receiver. Transmitter
functions typically include multiplexing, encoding, modulation, up-conversion

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from baseband or intermediate frequency (IF) to radio frequency (RF),
power amplification, and filtering for spectrum control.   Receiver functions
include RF filtering, down-conversion from RF to IF, amplification at IF,
equalization, demodulation, decoding, and demultiplexing.  To achieve point-
to-point radio links, antennas are placed on a tower or other tall structure at
sufficient height to provide a direct, unobstructed line-of-sight (LOS) path
between the transmitter and receiver sites.

Microwave System Design.  The design of microwave radio systems

involves engineer¬ing of the path to evaluate the effects of prop¬agation on
performance, development of a frequency allocation plan, and proper
selection of radio and link components. This design process must ensure that
outage requirements are met on a per link and system basis. The frequency
allocation plan is based on four elements: the local fre¬quency regulatory
authority requirements, selected radio transmitter and receiver
characteristics, antenna characteristics, and potential intrasystem and
intersystem RF interference. 

Microwave Propagation Characteristics.  Various phenomena associated

with propagation, such as multipath fading and interference, affect
microwave radio performance.  The modes of propagation between two radio
antennas may include a direct, line-of-sight (LOS) path but also a ground or
surface wave that parallels the earth's surface, a sky wave from signal
components reflected off the troposphere or ionosphere, a ground reflected
path, and a path diffracted from an obstacle in the terrain. The presence and
utility of these modes depend on the link geometry, both distance and
terrain between the two antennas, and the operating frequency. For
frequencies in the microwave (~2 – 30 GHz) band, the LOS propagation
mode is the predominant mode available for use; the other modes may
cause interference with the stronger LOS path. Line-of-sight links are limited
in distance by the curvature of the earth, obstacles along the path, and free-
space loss. Average distances for conservatively designed LOS links are 25
to 30 mi, although distances up to 100 mi have been used. For frequencies
below 2 GHz, the typical mode of propagation includes non-line-of-sight
(NLOS) paths, where refraction, diffraction, and reflection may extend
communications coverage beyond LOS distances. The performance of both
LOS and NLOS paths is affected by several phenomena, including free-space
loss, terrain, atmosphere, and precipitation.

Advantages

 Adapts to difficult terrain

 Loss versus distance (D)  = Log D (not linear)

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  Flexible channelization
 Relatively short installation time
 Can be transportable
 Cost usually less than cable
 No “back-hoe” fading

Disadvantages

 Paths could be blocked by buildings

 Spectral congestion
 Interception possible
 Possible regulatory delays
 Sites could be difficult to maintain
 Towers need periodic maintenance
 Atmospheric fading

Applications

The tremendous growth in wireless services is made possible today through

the use of microwaves for backhaul in wireless and mobile networks and for
point-to-multipoint  networks.  Towers can be used for both mobile, e.g.
cellular, and point-to-point applications, enhancing the potential for
microwave as wireless systems grow.  Increases in spectrum allocations and
advances in spectrum efficiency through technology have created business
opportunities in the field of microwave radio.   Telecommunications carriers,
utility companies, and private carriers all use microwave to complement
wired and optical networks.

ii. Radar

Basic Principle of Operation

The basic principle of operation of primary radar is simple to understand.

However, the theory can be quite complex. An understanding of the theory
is essential in order to be able to specify and operate primary radar systems
correctly. The implementation and operation of primary radars systems
involve a wide range of disciplines such as building works, heavy mechanical
and electrical engineering, high power microwave engineering, and advanced
high speed signal and data processing techniques. Some laws of nature have
a greater importance here.

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Radar measurement of range, or distance, is made possible because of the
properties of radiated electromagnetic energy.

1. Reflection of electromagnetic waves

The electromagnetic waves are reflected if they meet an electrically leading
surface. If these reflected waves are received again at the place of their
origin, then that means an obstacle is in the propagation direction.
 
2. Electromagnetic energy travels through air at a constant speed, at
approximately the speed of light,
o 300,000 kilometers per second or
o 186,000 statute miles per second or
o 162,000 nautical miles per second.

This constant speed allows the determination of the distance between the
reflecting objects (airplanes, ships or cars) and the radar site by measuring
the running time of the transmitted pulses.
 

3. This energy normally travels through space in a straight line, and will vary
only slightly because of atmospheric and weather conditions. By using of
special radar antennas this energy can be focused into a desired direction.
Thus the direction (in azimuth and elevation of the reflecting be objects can
measured.
 

These principles can basically be implemented in a radar system, and allow

the determination of the distance, the direction and the height of the
reflecting object.

(The effects atmosphere and weather have on the transmitted energy will be
discussed later; however, for this discussion on determining range and
direction, these effects will be temporarily ignored.)

Satellite Systems

Satellite communication systems consist of Earth-orbiting communications

platforms that receive and retransmit signals from earth-based stations. A
typical television satellite receives a signal from a base station and
broadcasts it to a large number of terrestrial receivers. Signals to satellites
are called "uplinks," and signals from satellites are called "downlinks."

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Uplinks have also been called "shooting the bird." The downlink covers an
area called the "footprint," which may be very large or cover a focused area.
Satellites use microwave frequencies. Since they are overhead, the
transmissions are line of sight to the receiver.

There are three different types of satellite systems.

    1. International satellite communication system INTELSAT.
    2. Domestic satellite system DOMSAT.
    3. Search and rescue system SARSAT.

INTELSAT:

    The INTELSAT Organization was established in 1964 to handle the myriad
of technical and administrative problems associated with a world wide
telecommunication system. The international regions served by INTELSAT
are divided in to the Atlantic Ocean region (AOR), the Pacific Ocean Region 
(POR),  and the Indian Ocean region (IOR). For each region , satellites are
positioned in geo-stationary orbit above the particular Ocean, where they
provide a transoceanic telecommunication route. In addition to providing
trans oceanic routes, the INTELSAT satellites are used for domestic services
within any given country and regional services between countries. Two such
services are vista for telephony and Intelnet for data exchange.

DOMSAT

    Domestic satellites are used to provide various telecommunication

services, such as voice, data, and video transmission (T.V channels), with in
a country.  

SARSAT

    SARSAT is one type of  Polar orbiting satellites.

    Polar-orbiting satellites orbit the earth in such a way as to cover the north
and south polar-regions. Infinite number of polar satellite orbits are
possible       

    Polar satellites are used to provide environmental data, and to help locate
ships and aircrafts in  distress .This service known as SARSAT, for search
and rescue satellite.

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 This figure shows polar satellite path and earth
rotation                                                                                           
     

Principle of Operation

A satellite communication system can be broadly divided into two segments,

a ground segment and a space-segment. The space system includes:

Power supply

   The primary electrical power for operating electronic equipment is

obtained from solar cells. Individual cells can generate small amounts of
power, and therefore array of cells in series-parallel connection are
required .

  Cylindrical solar arrays are used with spinning satellites,(The gyroscopic

effect of  the spin is used for  mechanical orientational stability) Thus the
array are only partially in sunshine at any given time. 

   Another type of solar panel is  the rectangular array or solar sail. solar sail
must be folded  during the launch phase and extended when in geo-
stationary orbit. Since the full component of solar cells are exposed to sun
light ,and since the Sail rotate to track, the sun , they capable of greater
power output than cylindrical arrays having a comparable number of cells.

   To maintain service during an eclipse, storage batteries must be provided .

Attitude control:

  The attitude of a satellite refers to its Orientation in space. Much of

equipment carried abroad a satellite is there for the purpose of controlling its
attitude. Attitude control is necessary, for example, to ensure that
directional antennas point in the proper directions. In the case of earth
environmental satellites the earth-sensing instrument must cover the
required regions of the earth, which also requires attitude control. A number
of forces, referred to as disturbance forces can alter attitude, some
examples being the gravitational forces of earth and  moon, solar radiation,
and meteorite impacts. 

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Station keeping:

  A satellite that is normally in geo-stationary will also drift in latitude, the

main perturbing forces being the gravitational  pull of the sun and the
moon . the force cause the inclination to change at the rate of about 0.85
deg./year. if left uncorrected, the drift would result in a cycle change in the 
inclination  going 0 to 14.67deg in 26.6 years and back to zero , when the
cycle is repeated. To prevent the shift in inclination from exceeding specified
limits, jets may be pulled at the appropriate time to return the inclination to
zero. Counteracting jets must be pulsed when the inclination is at zero to
halt that change in inclination.

Thermal control:

   Satellites are subject to large thermal gradients, receiving the sun

radiation on one side while the other side faces into space. In addition,
thermal radiation from the earth, and the earth's abedo,  which is the
fraction on the radiation falling on the earth which is reflected can be sight
for low altitude, earth-orbiting satellites, although it is negligible for geo-
stationary satellites. Equipment in the satellite also generates heat which
has to be removed. the most important consideration is that the satellite's
equipment should operate as near as  possible in a stable temperature
environment. various steps are taken to achieve this. Thermal  blankets and
shields may be used to provide insulation. radiation mirrors are often used
to remove heat from communication payload. These mirrored drums
surrounded the communication equipment shelves in each case and provide
good radiation paths for the generated heat to escape in to surround space.

    To maintain constant-temperature conditions, heaters may be switched

on to make up for the hearts may be switched  on to make reduction that
occurs when transponders are switched off. 

TT&C subsystem

Telemetry system

The telemetry, tracking, and command (TT&C) subsystem performs several

routine functions abroad a spacecraft. the telemetry or "telemetering" 
function could be interpreted as "measurement at a distance". specifically, it
refers to the over all operation of generating an electrical signal proportional
to the quantity being measured, and encoding and transmitting this to a
distant station, which for satellite is one of the earth stations, which for the

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satellite is one of the earth stations. Data that are transmitted as telemetry
signals include attribute information such as obtained from sun earth
sensors; environmental information such as magnetic field  intensity and
direction; the frequency of meteorite impact and so on ;and spacecraft
information such as temperatures and power supply voltages, and stored
fuel pressure. 

Command systems

Command system receives instructions from ground system of satellite and

decodes the instruction and sends commends to other systems as per the
instruction.

Tracking;

Tracking of the satellite is accomplished by having the satellite is

accomplished by having the satellite transmit beacon signals which are
received at the TT&C earth stations. Tracking is obviously important during
the transmitter and drift orbital phases of the satellite launch. When on-
station, a geo-stationary  satellite will tend to shifted as a result of the
various distributing forces, as described previously . Therefore it is necessary
to be able to track the satellites movements and send correction signals as
required. Satellite range is also required for time to time. This can be
determined by measurement of propagation delay of signals specially
transmitted for ranging purposes.

Transponders:

A transponder is the series of interconnected units which forms a single

communication channel between the receive and transmit antennas in a
communication satellite. Some of the units utilized by a transponder in a
given channel may be common to a number of transponders. Thus,
although  reference may be made to specific transponder, this must be
thought of as an equipment channel rather than single item of equipment.

Transponder consist of wideband receivers, input de-multiplexer, power

amplifier components. 

Antenna sub system:

The Antennas carried abroad a satellite provide the dual functions of

receiving the up link and transmitting the down link signals. They range from
dipole-type antennas, where omni directional characteristics are required, to

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the highly directional antennas required for telecommunications purposes
and TV relay and broadcasting.

References

o Digital Transmission Systems, Second Edition, by David R. Smith. 

Kluwer Academic Publishers, 2004.  Good introduction to microwave
systems, including modulation, error correction codes, equalization,
and diversity.
o Microwave Radio Links: Theory to Design by Carlos Salema.  John
Wiley and Sons, 2000.  Excellent treatment of all aspects of microwave
systems engineering